The present invention relates to hydraulic systems and pertains particularly to a hydraulic control system for a pipelayer.
Vehicles having a boom and winch assembly specifically designed for lifting and handling sections of pipe and the like for construction and laying of pipe are normally referred to as pipelayers. Such pipelayers commonly employ a winch for controlling the position of a boom and a winch for controlling the raising and lowering of a load suspended from the boom. Hydraulical motors are commonly employed for powering the winches of the boom and the load-lift assembly. These winches are normally powered by separate hydraulic systems or circuits. However, since the boom is commonly maintained in a preselected position for an extended period of time, it is desirable that hydraulic power available for powering the boom winch may also be utilized for assisting in powering the hoist winch in order to increase the speed thereof.
Hydraulic systems having provision for combining fluid from separate pumps are known in the art. However, special considerations are required in hoist systems which are not present in a typical hydraulic circuit. For example, load-responsive control means are provided for reducing the speed of the hoist motor and increasing the torque thereof upon encountering the predetermined load. Such combining should also be such that it does not interfere with the operation of the other system. Some examples in the known prior art are as follows: U.S. Pat. No. 3,208,221 issued Sept. 28, 1965 to Schuetz; U.S. Pat. No. 3,800,669 issued Apr, 2, 1974 to Distler; and U.S. Pat. No. 3,815,478 issued June 11, 1974 to Axelesson et al.
While these patents disclose systems that are of interest, they do not suggest a satisfactory system for solving the problems set forth.